NPC Summons function like any other summon. They'll help you fight against any and all enemies including invaders. Watching an NPC summon and an NPC invader duke it out is quite a spectacle. Oh, and most likely because NPC summon/invader AI is laughable at best, most are tanky as fuck. NPC invaders less so, however. Perfect example of a tanky NPC summon: Iron Tarkus. You will learn to love him soon.